Let It Snow, Let It Snow, Puzzled Scientists Say : Weather: Researchers are mystified at early melting trends. It’s too early to blame global warming, they say.
After slogging through it, being stuck in it and even dying from it, you’d think that human beings by now would know everything there is to know about snow. Not so.
Solid, liquid and gas at varying times, snow is a complex substance whose behavior--on land and in the air--still eludes scientists. Wet snow is an entirely different material from dry snow, and within the wet and dry categories are numerous different types.
When snow hits the ground, it is no longer the same substance that has been falling through the atmosphere. And fallen snow, a good insulator of the soil it blankets, undergoes constant change.
What happens to snow on the ground--how it bonds, breaks apart, melts and refreezes--matters for everything from avalanche prediction to the design of better-performing tires for military tanks and private cars, safer skis for airplanes, faster skis for people, more effective but environmentally correct de-icing chemicals for highways and “grooming” techniques for smoother, longer-lasting snow roads and snowmobile trails.
The end of the Cold War has melted some snow-cover research, says Russell Alger, director of the Institute for Snow Research in Michigan’s Upper Peninsula. Except for Bosnia-Herzegovina in winter, the big push now militarily is for improved vehicle mobility--not in snow, but in desert sand.
More than 95% of Earth’s seasonally snow-covered land lies in the Northern Hemisphere, which holds most of the planet’s landmass.
From December through March, the white stuff blankets 16 million to 20 million square miles of the hemisphere, the majority of the land north of 40 degrees latitude. By midsummer it all disappears, except for glacial snow and ice fields.
Since 1972, satellite images, which have provided the first consistent record of snow coverage, have shown that the year-to-year horizontal expanse of winter snow has not varied significantly across the Northern Hemisphere as a whole.
“But the snow has melted earlier in the spring in the past six years--in March and April, rather than April and May,” said geographer David A. Robinson, of Rutgers University in New Jersey, who has analyzed the satellite data.
“The biggest change in recent snow cover is not so much its lack in winter as its early end, its reduction in spring,” agreed Kenneth F. Dewey, professor of climatology at the University of Nebraska.
Why the early spring snow loss? Scientists aren’t sure.
“There is a strong correlation between temperatures in the Northern Hemisphere and the extent of snow cover,” Robinson said. “Does recent warmth in spring cause the early end of snow cover, or does the early snow loss have a subsequent impact on temperature? Once you have the loss, it kind of snowballs.”
Researchers are “trying to unravel this chicken-and-egg question,” Robinson said. “It is too early--the record is too short--to attribute this to global warming.”
While snow coverage has not changed much globally, snow depth may have decreased. There is no similar satellite record, because the current technology cannot yet distinguish different snow depths.
“Monitoring snow cover may give an index to future climate change,” Robinson said. “Snow has to be an important piece of the climatological puzzle.”
Earth loses a good portion of its heat from the reflection of sunlight off snow.
Snow on the ground, especially fresh snow (the whitest), reflects back about 90% of the sun’s rays, said Samuel C. Colbeck, a senior research scientist at the U.S. Army Cold Regions Research and Engineering Laboratory in Hanover, N.H. “The more snow cover, the more solar radiation is reflected back.”
To fit another piece into the weather puzzle, Dewey is analyzing satellite data of Southern Hemisphere snow cover for the first time to determine whether it is in phase with the Northern Hemisphere’s. About 800,000 square miles of the Southern Hemisphere, mostly in South America, are snow-covered seasonally.
